Power steering system

ABSTRACT

A power steering system for a motor vehicle comprising a hydraulic reaction device ( 11 ) by which reaction power is generated on the steering column of the vehicle, depending on the speed thereof. According to the invention, a accumulator ( 14 ) is provided on the line ( 13 ) connecting the reaction device ( 11 ). Said accumulator serves as a low-pass filter and decouples the reaction device from oscillations in pressure due to uneven road surfaces.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of German patent document 199 40026.1, filed Aug. 24, 1999, and PCT International Application No.PCT/EP00/07626, filed Aug. 4, 2000, the disclosures of which isexpressly incorporated by reference herein.

The invention relates to a power steering system having a hydraulicfeedback arrangement that generates a reaction force at the steeringcolumn.

German patent document DE 41 12 735 C2 discloses such a power steeringsystem, in which the steering reaction is generated as a function of thespeed of the motor vehicle, and can be felt by the driver of the motorvehicle as a steering resistance. In the connecting line of thisfeedback device there is a throttle for adjusting the characteristics ofthe power assist.

In motor vehicles of the current 5 and 7 series of the assignee of thisapplication, a power steering system, called “Servotronic”, is installedas optional equipment. Contrary to the power steering systems such as isknown from the German patent document DE 41 12 735 C2, in which thefeedback is the result of the amount of power assist, these motorvehicles of the assignee have a separate feedback device, where areaction force is applied to the steering shaft by way of a pressuredrop at a series connection of hydraulic resistors.

In known steering systems high frequency pressure surges, which generateonly a low volume flow, are not affected in principle by the hydraulicresistance. Consequently high frequency pressure fluctuations,generated, for example, when driving on rough roads, are passed largelyunfiltered into the feedback device and induce steering wheel torquejolts over both the working cylinder of the power steering system andthe hydraulic feedback device. Especially in the case of rack and pinionsteering systems with their comparatively low mass moments of inertia,road excitations lead to a “bumpiness” in steering.

One object of the invention is to provide an improved power steeringsystem of the type described above.

This and other objects and advantages are achieved by the power steeringsystem according to the invention, in which the vibrating behavior ofthe liquid column in the connecting line of the feedback device isinfluenced as a function of the frequency of the vibrations. To thisend, there is preferably a hydraulic low pass filter, which is effectiveto filter out jolts which have a frequency above that of typicallyintroduced steering movements (claim 6).

In one embodiment of the invention, the filter includes a hydrauliccapacitor, which is formed, for example, by a piston-type, bladder ordiaphragm accumulator. Furthermore, the hydraulic capacitor can also beimplemented by especially elastic walls of the oil-conducting chambersinside the steering system. Such a configuration can be achieved, forexample, with a hose connection. The hydraulic capacitor can be combinedwith a hydraulic resistor (constrictor or throttle) in seriesconnection, according to a preferred arrangement of the invention.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a power steering system according to the prior art; and

FIG. 2 depicts the power steering system according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The schematic drawing of FIG. 1 shows a power steering system for amotor vehicle, where the steering movement is assisted by a hydraulicworking cylinder 1. A power steering pump for pressurization withhydraulic oil is marked with the reference numeral 2. The hydraulic oilis taken from a supply tank 3 and recycled again into the same. Steeringmovements at a steering column (not illustrated) are passed into thepower steering system by means of a steering valve 4. The latter isconnected to a feedback device 11 by means of a connecting line 13, inwhich a transducer 5, formed as a solenoid valve, and hydraulicresistors 6 and 7 are located.

In the event of steering wheel movements the hydraulic resistors 6 and 7produce variable pressure differentials, with which a feedback piston 12is driven in the feedback device 11. The transducer 5 activates thefeedback device 11 as a function of the speed of the motor vehicle sothat there is a speed-dependent steering wheel torque feedback to thesteering column. A check valve 8 in the direction of the pressure sideof the power steering pump 2 minimizes vibrations in the power steeringsystem. The check valves 9 and 10 effect the desired activation of thefeedback device 11 as a function of the direction of movement of theworking cylinder 1, together with the transducer 5 and the hydraulicresistors 6 and 7.

Low frequency pressure fluctuations or stationary pressure ratios,stemming from intentionally initiated steering movements of the driver,generate a desired steering wheel torque feedback. Pressure fluctuationsof higher frequency, due for example to an uneven road surface, can beaffected only slightly by the hydraulic resistors 6 and 7, since thesehigher frequency pressure fluctuations induce only a very low volumeflow. Consequently the higher frequency pressure fluctuations are passedlargely unfiltered to the feedback device 11 and induce at the feedbackpiston 12 pressure differentials that result in steering wheel torquejolts at the steering gear input, and thus at the steering column.

As apparent from FIG. 2, the invention provides at the connecting line13 between the steering valve 4 and the feedback device 11 a hydraulicaccumulator 14, which acts as the hydraulic capacitor. The accumulator14 does not affect the low frequency pressure fluctuations, induced bythe steering movements, or the stationary pressure ratios and thus thesteering behavior of the motor vehicle. In contrast, however, theaccumulator 14 uncouples disturbing higher frequency pressurefluctuations from the feedback device 11. This desirable feature isachieved by appropriately tuning the accumulator 14 to the powersteering system, especially the hydraulic resistors 6 and 7; as aresult, higher frequency pressure fluctuations queue at the resistors 6and 7 with a reduced pressure gradient, owing to the smoothing effect ofthe accumulator 14 and generate there a correspondingly lower pressuredrop and thus also lower throughflow.

The described “low pass effect” of the accumulator 14 specificallyfilters out the road-induced jolts, which, in addition to the directmechanical introduction over the steering linkage, are passed into thesteering system over a hydraulic path of action. In this manner thehydraulic path for introducing vibrations is specifically damped, sothat steering comfort is improved by preventing or significantlyreducing the introduction of vibrations.

Known measures for reducing the jolts, for example by means of checkvalves 8 in the pressure line between the power steering pump 2 and thesteering valve 4 or a mechanical steering wheel shock absorber are lesseffective in this respect, since these methods cannot prevent hydraulicfeedback over the feedback device 11.

The threshold frequency, at which the low pass filter acts, preferablylies below the frequency of the natural vibration of the axles thatranges from approximately 12 to 20 Hz in passenger cars. In other typesof motor vehicles, the natural frequency of the axles can also occurdown to and into the range of 5 Hz. On the other hand, the low passfilter cannot negatively affect pressure fluctuations that stem fromintentional steering wheel movements and reach a frequency of up toabout 2 Hz.

According to the embodiment of the invention, according to FIG. 2, theaccumulator 14 is disposed in a blind line between the transducer 5 andthe hydraulic resistors 6 and 7 and is thus formed preferably as adiaphragm or bladder accumulator. Depending on the design of the powersteering system, however, it is also possible to integrate theaccumulator 14 as an expansion hose, provided with elastic walls,directly into the connecting line 13 between the transducer 5 and thehydraulic resistors 6 and 7, thus forming the accumulator 14 as thethroughflow accumulator.

As also depicted in FIG. 2, in addition to the accumulator 14, ahydraulic resistor 15 can be connected in series, making it possible tofurther tune the low pass filter. In so doing, primarily the degree ofdamping and the gradient of the pressure buildup are affected, whereasthe dimensioning of the threshold frequency follows automatically fromthe layout of the accumulator 14.

The distinction between the arrangement of the accumulator 14 (and thehydraulic resistor 15) downstream of the transducer 5 and other possiblearrangements lies in its especially high efficiency.

Preferred fields of application of the invention are rack and pinionsteering systems, where road-induced jolts are damped only inadequatelyowing to the lower mass moments of inertia of the individual componentsof the steering system. Of course, the invention can be used in alltypes of steering systems, for example in recirculating ball steeringsystems. The additional cost for an accumulator 14 and optionallyanother hydraulic resistor 15 is comparatively low, since the saidcomponents are standard components.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A power steering system for a motor vehicle,having a hydraulic feedback device connected by a feedback connectionline to a steering mechanism of the vehicle, for producing a steeringreaction at the steering column of the motor vehicle, comprising: adevice connected in pressure flow communication with the feedbackconnecting line, for influencing a fluid column in the feedbackconnecting line, as a function of a frequency of vibration of the fluidcolumn; the device for influencing the fluid column comprises at leastone hydraulic low pass filter; and a cutoff frequency of the low passfilter lies above a frequency of steering movements introduced by adriver of the motor vehicle.
 2. The power steering system according toclaim 1, wherein the device for influencing the fluid column comprisesat least one hydraulic low pass filter.
 3. The power steering systemaccording to claim 1, wherein the device for influencing the fluidcolumn includes a hydraulic resistor.
 4. The power steering systemaccording to claim 1, wherein the device for influencing the fluidcolumn includes a hydraulic capacitor.
 5. The power steering systemaccording to claim 4, wherein the device for influencing the fluidcolumn includes a hydraulic resistor.
 6. The steering system accordingto claim 1, wherein the device for influencing the fluid column isdisposed between a transducer, which controls a rate of fluid flow inthe feedback connection line as a function of speed of the motorvehicle, and the feedback device.
 7. The power steering system accordingto claim 2, wherein a cutoff frequency of the low pass filter lies abovea frequency of steering movements introduced by a driver of the motorvehicle.
 8. The power steering system according to claim 4, wherein thehydraulic capacitor comprises at least a section of a hose connection ofthe steering system, which section has walls whose elasticity ispreselected to achieve a desired damping effect.
 9. The power steeringsystem in accordance with claim 1, wherein said cutoff frequency is setat approximately two Hz.
 10. A vehicle steering mechanism, comprising: aworking cylinder for assisting steering of vehicle wheels; a steeringvalve for applying pressure to the working cylinder in response tosteering inputs by a vehicle operator; a feedback device fortransmitting reaction forces in response to pressure transmitted fromsaid steering valve; a feedback connection line connecting said steeringvalve in pressure flow communication with said feedback device; and ahydraulic pressure accumulator connected in pressure flow communicationwith said feedback connection line; wherein the hydraulic pressureaccumulator comprises at least a hydraulic low pass filter having acutoff frequency that is above a frequency of steering movementsintroduced by a driver of the vehicle.
 11. A vehicle steering mechanismaccording to claim 10, wherein said hydraulic pressure accumulator is ahydraulic low pass filter.
 12. A vehicle steering mechanism according toclaim 10, wherein said low pass filter further includes a hydraulicresistor.
 13. A vehicle steering mechanism according to claim 10,wherein the hydraulic pressure accumulator comprises at least a sectionof a hose connection of the steering system, which section has wallswhose elasticity is preselected to achieve a desired damping effect. 14.The vehicle steering mechanism according to claim 10, wherein thehydraulic pressure accumulator is disposed between a transducer whichcontrols a rate of fluid flow in the feedback connection line as afunction of vehicle speed, and the feedback device.
 15. The powersteering system in accordance with claim 10, wherein said cutofffrequency is set at approximately two Hz.